The present disclosure generally relates to semiconductor implantation, and more specifically, to methods for selectively implanting polyhedral boranes or its lower order byproducts into semiconductor work pieces.
Conventional ion implantation systems, used for doping work pieces such as semiconductors, include an ion source that ionizes a desired dopant element, which is then accelerated to form an ion beam of prescribed energy. The ion beam is directed at the surface of the workpiece to implant the workpiece with the dopant element. The energetic ions of the ion beam penetrate the surface of the workpiece so that they are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity. The implantation process is typically performed in a high-vacuum process chamber which prevents dispersion of the ion beam by collisions with residual gas molecules and which minimizes the risk of contamination of the workpiece by airborne particulates.
Ion dose and energy are the two most important variables used to define an implant step for a particular species. Ion dose relates to the concentration of implanted ions for a given semiconductor material. Typically, high current implanters (generally greater than 10 milliamps (mA) ion beam current) are used for high dose implants, while medium current implanters (generally capable up to about 1 mA beam current) are used for lower dose applications.
Ion energy is used to control junction depth in semiconductor devices. The energy of the ions that make up the ion beam determines the degree of depth of the implanted ions. High energy processes such as those used to form retrograde wells in semiconductor devices require implants of up to a few million electron-volts (MeV), while shallow junctions may only demand energies below 1 thousand electron-volts (keV), and ultra-shallow junctions may require energies as low as 250 electron-volts (eV).
The continuing trend to smaller and smaller semiconductor devices requires implanters with ion sources that continue to deliver higher beam currents at lower energies. The higher beam current provides the necessary dosage levels, while the lower energy levels permit shallow implants. Source/drain junctions in complementary metal-oxide-semiconductor (CMOS) devices, for example, require such high current, low energy applications.
Conventional ion sources utilize an ionizable dopant gas that is obtained either directly from a source of a compressed gas or indirectly from a vaporized solid. Typical source elements are boron (B), phosphorous (P), gallium (Ga), indium (In), antimony (Sb), and arsenic (As). Most of these source elements are commonly used in both solid and gaseous form, except boron, which is almost exclusively provided in gaseous form, e.g., as boron trifluoride (BF3), or as a compound in solid (powder) form as decaborane (B10H14).
Decaborane (B10H14) could be an excellent feed material for boron implants because each decaborane molecule (B10H14) when vaporized and ionized can provide a molecular ion comprised of ten boron atoms. Such a source is especially suitable for high dose/low energy implant processes used to create shallow junctions, because a molecular decaborane ion beam can implant ten times the boron dose per unit of current as can a monatomic boron ion beam. In addition, because the molecular weight is also 10 times that of a monotonic ion, the decaborane molecule must be accelerated to 10 times the energy in order to achieve the same implanted depth in the target workpiece. (The individual boron atoms of a singly charged decaborane molecule (B10HX+) of 10 identical boron atoms accelerated with a voltage V, each have an energy of eV/10, and thus the ion beam will be extracted at 10 times the required energy). This feature enables the molecular ion beam to avoid the transmission losses that are typically brought about by low-energy ion beam transport and so called “space charge” effects.
Recent process and ion source improvements have enabled the generation of ion beam currents that might prove in the future to be sufficient for production applications of decaborane implants. Keys to such improvements are ion source operation at relatively low temperatures which prevents dissociation of the decaborane molecule and fragmentation of the desired parent molecular ion (B10HX+) into borane fragments and elemental boron. In addition, in known decaborane ion sources, such as that shown in U.S. Pat. No. 6,107,634, a low-density plasma is maintained to prevent the plasma itself from causing such dissociation and fragmentation.
As stated above, future ultra shallow junctions in semiconductors will likely require boron implants with implant energies as low as 100 electron volts (eV). At such low energies, ion beam current densities will necessarily decrease. In addition, semiconductor implant throughput will decrease due to the transport issues at low energy. Alternatively, it would be desirable to increase the ion beam energy transport levels without increasing the energy levels of the individual boron atoms implanted. This can be accomplished through the use of deceleration or the utilization of higher order ionized molecules.